Trough hearth construction and method for plasma arc furnace

ABSTRACT

An improved hearth construction and method are disclosed for plasma arc furnaces adapted for melting metallic raw or scrap materials and are based on using a shaped receptacle, e.g., an annular-shaped trough, integrally molded into the furnace hearth which enables molten metallic materials to rapidly accumulate and the molten accumulated materials to be contacted by long arc plasma columns very early in the melting procedure, thereby substantially reducing the damage to torch parts due to arc flashing and improving the operating efficiency.

- United States Patent 1 Camacho July 31, 1973 I 1 TROUGH HEARTH CONSTRUCTION AND METHOD FOR PLASMA ARC FURNACE [75] Inventor: Salvador L. Camacho, Raleigh, NC.

[73] Assignee: Technology Application Services Corporation, Raleigh, NC.

[22] Filed: Aug. 24, 1972 [21] Appl. No.: 283,552

[52] US. Cl 13/9, 13/1, 13/35 [51] Int. Cl. 1105b 7/18 [58] Field of Search 13/1, 2, 9, 31, 34, 13/35; 219/121 P [56] References Cited UNITED STATES PATENTS 3,406,241 10/1968 Robinson 13/9 X 3,422,206 1/1969 Baker et a1. 13/34 8/1969 Estes 13/9 5/1971 Schoumaker 219/121 P Primary Examiner-Roy N. Envall, Jr. Attorney-B. 8. Olive [5 7 ABSTRACT An improved hearth construction and method are dis closed for plasma arc furnaces adapted for melting metallic raw or scrap materials and are based on using a shaped receptacle, e.g., an annular-shaped trough, integrally molded into the furnace hearth which enables molten metallic materials to rapidly accumulate and the molten accumulated materials to be contacted by long are plasma columns very early in the melting procedure, thereby substantially reducing the damage to torch parts due to are flashing and improving the operating efficiency.

13 Claims, 8 Drawing Figures BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to electric arc furnaces adapted for melting raw or scrap metallic materials and especially to those furnaces employing long are column forming plasma torches. I

2. Description of the Prior Art Plasma column torches, per se, are taught, for example, in U. S. Pats. Nos. 3,194,941 and 3,673,375. Electric arc furnace prior art includes U. S. Pat. No. 1,479,662 which broadly teaches the utilization of an annular trough in a carbon arc type electric furnace. The trough, as taught in this reference, is adapted to reside proximate the furnace interior vertical wall concentric to a bottom and centrally located carbon electrode. Although admittedly quite old in the art, such a trough configuration has never, so far as is known, been heretofore used with a long arc column forming plasma torch. More specifically, such an annular trough has never been used in a configuration which allows the molten material inthe trough to form part of the arc circuit.

Other notable prior art includes Baker et al. U. S. Pat. No. 3,422,206, which teaches the use of sidewall mounted plasma torches in a furnace, as opposed to conventionally top mounted arc forming electrodes, as a means to promote greater furnace efficiency. Such furnaces employing side mounted plasma torches, while constituting a significant improvement in furnace efficiency by minimizing heat losses, have still been characterized by certain degrees of inefficiency by what may be characterized as are flashing before flat bath, i.e., a total molten state of the charge, has been achieved. Arc flashing may be defined as random electrical contact of a plasma arc column with metallic scrap not in alignment with the axis of the arc forming plasma torch. Melting of scrap material normally .proceeds as a plasma arc column is established between a metallic scrap in axial alignment with the plasma torch. The two critical factors contributing to arc flashing" now come into play. These are depth of melting into the scrap (0,), and the diameter of the path formed through the scrap (D,). It has been experimentally recognized by this inventor and by others skilled in the art that when the ratio D /D, becomes greater than 1, the arc column becomes unstable and transfers or flashes to the closest point of electrical contact which affords the least electrical resistance. This point is typically an outer edge of the hole which has been melted in the scrap. This action results in bending of the arc column from its normal axis, contacting of cooler surfaces by the arc and consumption of more power than would otherwise be required. In addition,

The above cited Baker et al. patent does not suggest a means for reducing the characteristic arc flashing of the plasma arc columns randomly arcing between various bits of metal until a flat bath or totally molten fluid state has been reached. While side mounting of plasma torches has been taught, side mounting of long are column forming plasma torches in combination with a furnace hearth having an annular or equivalent shaped trough has not been suggested. Nor has such a combination been suggested as a means for electrically I I connecting the various torches being utilized to the same electrical circuit in order to speed melting, eliminate arc flashing, andtherebyobtain even greater oper ating efficiencies.

SUMMARY OF THE INVENTION No. 3,422,206. Various embodiments are described. In

the preferred embodiment of the present invention, however, there isemployed a furnace hearth having an' annular-shaped trough integrally molded therein. The furnace hearthis further adapted by sloping interior floor portions lying concentrically adjacent the annular trough to cause molten metallic materials to flow into the trough. f

In the present invention it has been discovered that an exceptionally long transferred plasma arc can best be established and maintained in a furnace of the class described, when melting metallic scrap or raw material, by collecting molten metal in the annular trough early into the melting procedure and electrically connecting each transferred plasma arc with the molten metal. In so doing, the previously mentioned depth/diameter D /D, relation has been found to no longer hold'true. The least resistive electrical path, and thus the one which the plasma transferred arc column will seek to follow, has been found to be the path between each respective torch andthe trough of molten metal.

Operation of a plasmaarc furnace according to the invention proceeds more rapidly and smoothly to flat bath and in the near absence of destructive arc flashing, while at the same time consuming substantially less power than conventional plasma arc furnaces whic employ flat or concave hearth floors.

DESCRIPTION OF THE DRAWINGS FIG. 1 is a vertical cross-sectional view of an electric furnace utilizing long are column forming plasma torches showing a trough hearth construction according to the invention. i

FIG 2 is a top plan view of an electric furnace according to the invention showing the relation of side mounted plasma torches to th e trough hearth configuration.

FIG. 3 is a somewhat schematic diagram showing. a power circuit adapted for use by the invention.

FIGS. 4, 5, and 6 are partial front views in crosssection of the furnace showing a long are column forming plasma torch in various positions indicating subsequent stages of melting raw metallic materials in accordance with the invention.

FIG. 7 is a front view in cross-section of a first alternate trough embodiment of the invention.

FIG. 8 is a top plan'view of an electric furnace according to the invention showing a second alternate trough embodiment.

DESCRIPTION OF THE PREFERRED EMBODIMENT In the description which follows, reference will be made to a plasma generator which has been defined in U. S. Pat. No. 3,673,375, entitled Long Arc Column Plasma Generator and Method, issued to same inventor, as an apparatus adapted to generate 'a long hightemperature plasma are between the apparatus and an Referring now to FIG. 2 which shows a somewhat from the radius R in order to achieve slight stirring of electrical conductor in the arc circuit. Such an are device comprises a cylindrical shaped electrode; a gasdirecting nozzle axially aligned with, forwardly spaced and insulated from said electrode; and appropriate gas, water and electrical supply means. The plasma generator or torch disclosed in U. S. Pat. No. 3,673,375 is highly suited to the needs of the present invention due to its ability to provide an exceptionally long transferred arc. Other plasma torches having a somewhat shorter arc capability may be adapted for use by the invention. See, for example, Baird, U. S. Pat. No. 3,194,941. Reference will also be made to the term raw metallic materials which, for purposes of the disclosure, is defined as any material capable of conducting electric current at least in the fused state and includes metals, metal alloys, fused salts, slags, etc.

Referring now to FIG. 1 of the drawings, a portion of an electric furnace generally designated 10 constructed according to known furnace techniques includes side 11 and bottom 12 wall portions, and a hearth 15 formed of molded refractory material. In the preferred invention embodiment, the furnace sidewalls 11 are provided with a plurality of suitable apertures 16 each being adapted to receive a long are column forming plasma torch 20. A cylindrical collar 22 is adapted to line each aperture 16 and receive each torch in sealingly movable engagement. Angular adjustment of each torch with respect to the sidewalls ll is'provided by appropriate remotely controllable jack means 24, while depth of torch insertion is controlled by suitable motorized carriage means 27. Lateral angular movement of each torch is provided by a movable support member 29 which supports jack 24 and carriage 27.

A hearth in such a furnace has a conventionally comprised a somewhat concave interior floor 26 having a lowermost point at the hearth center 28. In what is considered the heart of the present invention, an annular trough 30 is formed integral with such a concave hearth floor 26. Such trough is utilized in the instant invention, as will later be described in greater detail, for collecting a ring of molten metal early into the melting of raw metallic materials. In conventional furnaces having concave interior hearth floors, the metal drippings which occur early in the melting sequence are adapted toflow toward the center of the hearth. In the present invention, however, trough 30 is adapted to be approximately medially located between the hearth center and the furnace sidewall thereby collecting metal drippings before they reach the hearth center 28. The distance of trough 30 from the sidewall should not be so great so as to prevent electrical contact between long are (10 -40 inches) plasma columns, not shown, being formed by said torches 20 and molten metal, not shown, collected in the trough.

molten metal collected in trough 30. In plasma arc furnaces of the prior art stirring was recognized to occur at or near the time flat bath was reached by slightly angling the torches utilized away from the furnace radius. It is a recognized advantage of this invention to achieve stirring from very early in the melting sequence up until flat bath. thereby creating amore uniform and homogeneous molten charge. As represented by dashed lines 35, each torch 20 may be laterally angled, by previously mentioned means, toward the radius R during the first few minutes of furnace operation to utilize the most direct arc pathway to the molten metal being collected in trough 30, then swung slightly away from the radius to begin stirring.

Referring now to FIG. 3, each long are column forming plasma torch 20 utilized in the present invention (shown in cross section) includes a cylindrical shaped internal electrode 40 and a gas directing nozzle4l axially aligned with, forwardly spaced and insulated from electrode 40, and appropriate gas, water and electrical supply means. While the last mentioned gas and water supply means are considered well-established in the art requiring no further elaboration herein, a better understanding may be gained of the invention by a discussion of its use of power in starting and maintaining the various long are plasma columns. A three phase A. C. wye connected source 44 having a floating neutral 45 supplies power to three long are column forming plasma torches 20. Each torch internal electrode 40 is connected across an individual phase or leg A, B or C of the wye source 44. It is apparent that if more than three torches are employed a greater number of phases may be utilized. Each torch nozzle 41 is connected to an adjacent leg or phase via an appropriatechoke such as indicated at 46, 47 and'48. In this manner a pilot are 49 may be struck between the nozzle 41 and internal electrode of each torch withoutintroducing a consumable electrical conductor by applying the phase to adjacent phase voltage differential, caused by the respective choke, across internal electrode 40 and nozzle 41 and'by simultaneously introducing a gas vortical flow through nozzle 41. The current in the pilot arc is substantially smaller than that of the main are which is subsequently adapted to be transferred to the metal scrap to be melted. Simultaneous transferral of at least two pilot arcs" 49, 49 to the raw metallic materials establishes at least two main transferred arcs (not shown) and completes a phase B to adjacent phase C loop whereby thefull available current is now. utilized. Other pairs of arcs act similarly.

Referring next to FIGS. 4, 5 and 6 which illustrate subsequent stages during the melting of raw metallic materials 51 charged into the furnace l0, and which also illustrates an important new relationship serving as a basis for this invention, each long are column-forming plasma torch 20 is slightly extended as necessary into the furnace during the main arc striking procedure. When a pilot arc comes within 4-71- inches of ametal scrap 51 in the electrical circuit, transfer is made to a main are 50. This operation normally occurs simultaneously with all three torches being electrically connected. Simultaneous electrical connection of at least two torches is necessary for proper furnace operation according to this invention. Once the main transferred long are column 50 has been established, as shown in FIG. 4, a pathway 53 in axial alignment with the longitudinal axis X of torch 20 is begun to be melted in the a formed in the hearth in axial alignment with the respecraw metallic materials 51 and molten drippings 54 now begin to accumulatein trough 30. Referring to FIG. 5, as melting proceeds, within the first few minutes of furnace operation torch 20 is slightly withdrawn to form a longer transferred arc column 50, thereby radiating more heat, while melting a wider and deeper path 53. At this time, two dimensions come into play, the ratio of which has been experimentally determined to be critical. These are depth of path D and diameter of path D,. Up to a point, D, will increase at roughly the same rate as D, with D, being greater, then D, will increase more rapidly than D,. As the ratio of D lD, begins to reach 1 arc flashing or bending of the arc column to contact metal surfaces not in the axial arc pathway will occur. At this stage arc flashing is not damaging because very significant bending does not occur. When D /D, greatly exceeds 1, however, very significant bending of the arc column will occur resulting in arc flashing, and often leading to the'previously mentioned double arcing in which the torch nozzle may be damaged. As a recognized object, the instant invention substantially reduces the harmful arc flashing occurring as D /D, becomes significantly greater than I, by providing a molten pool of metal at that stage and in axial alignment with the torch, to which the long transferred plasma arc may become connected. Al-- though further from the torch 20 the molten pool of metal 54 collected in trough 30 offers much less resistance than surrounding cooler scrap surfaces nearer to torch 20 but not in axial alignment, and therefore attracts and holds the arc column 50 (best shown in FIG. 6). By collecting and forming a continuous trough of molten metal early in the melting procedure, all torches being utilized may now function at maximum efficiency. The result is a higher quality, more homogeneous bath due to the constant stirring from early in the melting sequence to flat bath, at a substantial savings in power consumed and torches damaged, due to the virtual elimination of harmful are flashing. It is recognized that the ratio D /D, will vary between raw metallic materials, andany trough according to the invention will be located with respect to the torches, with this consideration in mind. On the average the trough should be at a distance where D /D, has been found to approximately 1 for the materials normally being melted.

Referring now to FIG. 7, an alternate embodiment according to the invention utilizes the previously described conventional concave hearth floor 26 but provides amedially circular ridge 60 located between the sidewall 11 and hearth floor center 28'thereby effec tively forming a trough 30' outwardly and concentrically adjacent ridge 60, accomplishing the same invention objectives. Such a ridge type construction may find particular application in existing furnaces being refitted for long are column forming torches, but wherein the hearth has become substantially hardened through prolonged use, to preclude the easy formation of a trough 30 as shown in FIG. 1.

Referring to FIG. 8, a second alternate embodiment of the invention includes pockets or depressions 70 tive torches utilized and being adapted to collect molten raw metallic material to accomplish the aforementioned invention objects. To complete electrical connection between the adjacent depressions 70, small troughs indicated by dashed lines 71 may be provided. As is apparent from the foregoing invention embodiments, the depressions 70 may be formed in any suitable configuration down into thehearth floor 26 and may be at least partially surrounded by semicircular ridges indicted by dashed lines 65 in FIG. 8. Of course, with a concave floor such semicircular ridges 65 could,

I by themselves, be used as the melt collecting receptamain arc columns of at least two of. the torches during cles.

In accordance with the invention it is contemplated that such a method and apparatus as are disclosed herein will find wide application in either batch or continuous type melting operations. The art is sufficiently established as to the parameters of either operation so as to not warrant further discussion herein.

Summarizing what has been stated above, a generalized statement of the method of the invention includes:

. a. charging a furnace of the class described with raw metallic materials;

b. initiating and sustaining from a multi-phase power source at least two long plasma arc columns in axial alignment with and produced by at least two long arc column forming plasma torches;

melting the materials by use of the columns and providing a trough configuration for collecting metal drippings in axial alignment with said torches and at a distance from said torch whereby the ratio of depth of path melted to diameter of path melted is kept approximately 1 for the material being melted;

d. contacting the molten metal pool formed in said trough configuration with at least two of the said long arc plasma columns, whereby phase-to phase electrical connection is formed between said torches via said molten metal pool with the torches angled to cause the columns to strike the pool in a minimum distance; and v e. then horizontally angling the torch after melting has been initiated to facilitate stirring.

On the basis of the above, it is apparent that the instant invention provides an effective method and apparatus for melting and stirring raw metallic materials in an electric furnace of the class described through the utilization of an annular trough or equivalent configuration together with long arc column forming plasma torches so as to consume substantially less power than conventional furnaces and produce a higher quality, more homogeneous molten bath. The elongated pool of melt initially formed may take any of various forms so long as it is positioned so asito be contactable by the the initial melting. Once a relatively deep pool has been formed, all of the torches may, of course, maintain contact through the pool independent of the initial configuration of the melt. Axial, vertical and lateral positioning of all the torches may take place simultaneously or independently according tothe nature of the material and melting conditions encountered. In some furnace applications the torches may be found to operate successfully in fixed positions directed at the particular hearth depression. In other applications not all of the three forms of motion, i.e., axial, vertical and lateral,

may be required for torch positioning. However, it can be seen that in the initial starting of the melting procedure the invention both in an apparatus as well as a method sense offers advantages not heretofore recognized or achieved in the art. Of special significance is the use of a long are plasma column torch or generator by which is meant a plasma column generally in the 10 to 40 inches range but possibly longer and drawing relatively high current in the range, for example, of 1,000 amperes or more. It is this type plasma generator when combined with the described hearth that brings about the mentioned advantages.

What isclaimed is:

l. A method of melting raw metallic materials in an electric furnace having at least two wall mountd peripherally spaced long are column forming plasma torches energized through interconnected phases of a multi-phase electric source and having a hearth said hearth including a depression adapted to collect an elongated pool of molten metal, comprising:

a. charging the furnace with raw metallic material;

b. energizing said torches from said electric source and establishing for each torch a main arc plasma column in circuit with said material;

c. as melting proceeds positioning the torches such that the columns are directed to contact the said depression at selected points spaced from the respective torch nozzles whereby the ratio of depth of path melted to diameter of path melted approximates 1 for the material being melted;

d. collecting the molten in said depression to provide said pool of molten metal and transferring said main arcs to contact said pool of metal whereby to establish respective long arc columns which include at least a portion of said pool of metal in circuit therewith; and

e. thereafter continuing to melt said material while maintaining said long arc columns in contact with and in circuit with the melt.

2. The method as claimed in claim 1 wherein each said torch is angled downwardly towards the hearth during initial operation whereby each respective column thereof contacts said depression in a minimum distance, rapidly melts a path in said material and upon sufficient melt collecting in said depression each said column contacts and includes said melt in electrical circuit therewith, and then angling each said torch while its respective column maintains contact with said melt in the depression in a manner to cause stirring of the melt in said depression during the early stages of melting said material.

3. The method of claim 1 including the step of positioning said torches through means remote to the furnace.

4. The method of claim 2 including the step of positioning said torches through means remote to the furnace.

5. The method of claim 1 wherein at least three said torches are employed.

6. The method of claim 1 wherein said depression takes the form of an annular depression in the hearth floor spaced inwardly from the furnace sidewall and said torches and columns are respectively directed towards and contact the melt in said ann'ular depression. 7. The method of claim 1 wherein said positioningis effected axially, laterally and vertically by means remote from said furnace.

8. The method of claim 1 wherein said elongated pool collected in said depression is of sufficient length and positioned on the hearth floor so as to be contactable by the said main arc columns of at least two of said torches during initial melting of said material. v t

9. An electric furnace adapted'for melting raw metallic materials and having side and bottom walls and a plural number of apertures extending through portions of said sidewalls comprising, in combination:

a. a plurality of long are column forming plasma torches individually mounted in said apertures and adapted for adjustable positioning therein;

b. positioning means adapted to adjustably position each said torch with respect to the said hearth; 20 c.*a hearth forming the said furnace bottom wall and including a depression therein for collecting an elongated pool of molten metal disposed such that I upon appropriate torch positioning a distance between each respective torch nozzle and the said pool may be maintained such that the ratio of depth to diameter of melted path being fonned approximate l for the material being melted; and

d. electrical circuit means connected to said torches and adapted to initiate a pilot arc in each said torch enabling the creation from each pilot arc of a main arc column for each torch, with each respective column including said material in electrical circuit therewith, subsequently enabling the transfer of each main arc column to the melt in' said pool to form respective long arc columns'passing through respective open melted paths free of said material and of substantially less resistance than provided by paths through the said material. 7,

10. A furnace as claimed in claim 9 including control means adapted to remotely effect said positioning.

11. A furnace as claimed in claim 10 wherein said control means are adapted to effect said positioning axially, laterally and vertically.

12. A furnace as claimed in claim 9 wherein said depression is annular in form and whose collected melt is adapted to electrically connect at least two said long are transferred columns. g i g I 13. A furnace as claimed in claim 9 wherein said electrical circuit means provides a multi-ph'ase A.C. source and includes electrical connection of an internal electrode of each said torch being utilized with a sepa rate phase of said source and electrical connection of a nozzle portion of each said torch with an adjacent phase of the source via an electrical choke wherebya potential may be established between, said' internal electrode and said nozzle of each said torch to initiate said pilot arc, and whereby said pilot arc may be transferred to a proximate raw metallic material to establish a direct phase-to-phase potential between at least two torches having main arc plasma columns in electrical contact with the said raw metallic material.

I "CERTIFICATE OF "CORRECTION" fit Crt i'f idvtha t) errbfa peafs h v fit' ied Patent nd that said Le'tter s Patent are hereby corrected; s shownbelomnifi 5? a l should-appear, after the mew L R-,.JR; *wr- 3:1 1: TEGTMEYER 1 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent N D t d 3].,

lnventorbs) Salvador L. Camacho It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected. as shown below:

Column 7, line 31, the Word -metalshould appear after the Work "molten" Signedand sealed this 1st day of January 197E.

(SEAL) Attes t: v

EDWARD M.FLETCHER,JR. D; TEGTMEYER Attesting Officer Acting Commissioner of Patents 

1. A method of melting raw metallic materials in an electric furnace having at least two wall mounted peripherally spaced long arc column forming plasma torches energized through interconnected phases of a multi-phase electric source and having a hearth said hearth including a depression adapted to collect an elongated pool of molten metal, comprising: a. charging the furnace with raw metallic material; b. energizing said torches from said electric source and establishing for each torch a main arc plasma column in circuit with said material; c. as melting proceeds positioning the torches such that the columns are directed to contact the said depression at selected points spaced from the respective torch nozzles whereby the ratio of depth of path melted to diameter of path melted approximates 1 for the material being melted; d. collecting the molten in said depression to provide said pool of molten metal and transferring said main arcs to contact said pool of metal whereby to establish respective long arc columns which include at least a portion of said pool of metal in circuit therewith; and e. thereafter continuing to melt said material while maintaining said long arc columns in contact with and in circuit with the melt.
 2. The method as claimed in claim 1 wherein each said torch is angled downwardly towards the hearth during initial operation whereby each respective coluMn thereof contacts said depression in a minimum distance, rapidly melts a path in said material and upon sufficient melt collecting in said depression each said column contacts and includes said melt in electrical circuit therewith, and then angling each said torch while its respective column maintains contact with said melt in the depression in a manner to cause stirring of the melt in said depression during the early stages of melting said material.
 3. The method of claim 1 including the step of positioning said torches through means remote to the furnace.
 4. The method of claim 2 including the step of positioning said torches through means remote to the furnace.
 5. The method of claim 1 wherein at least three said torches are employed.
 6. The method of claim 1 wherein said depression takes the form of an annular depression in the hearth floor spaced inwardly from the furnace sidewall and said torches and columns are respectively directed towards and contact the melt in said annular depression.
 7. The method of claim 1 wherein said positioning is effected axially, laterally and vertically by means remote from said furnace.
 8. The method of claim 1 wherein said elongated pool collected in said depression is of sufficient length and positioned on the hearth floor so as to be contactable by the said main arc columns of at least two of said torches during initial melting of said material.
 9. An electric furnace adapted for melting raw metallic materials and having side and bottom walls and a plural number of apertures extending through portions of said sidewalls comprising, in combination: a. a plurality of long arc column forming plasma torches individually mounted in said apertures and adapted for adjustable positioning therein; b. positioning means adapted to adjustably position each said torch with respect to the said hearth; c. a hearth forming the said furnace bottom wall and including a depression therein for collecting an elongated pool of molten metal disposed such that upon appropriate torch positioning a distance between each respective torch nozzle and the said pool may be maintained such that the ratio of depth to diameter of melted path being formed approximate 1 for the material being melted; and d. electrical circuit means connected to said torches and adapted to initiate a pilot arc in each said torch enabling the creation from each pilot arc of a main arc column for each torch, with each respective column including said material in electrical circuit therewith, subsequently enabling the transfer of each main arc column to the melt in said pool to form respective long arc columns passing through respective open melted paths free of said material and of substantially less resistance than provided by paths through the said material.
 10. A furnace as claimed in claim 9 including control means adapted to remotely effect said positioning.
 11. A furnace as claimed in claim 10 wherein said control means are adapted to effect said positioning axially, laterally and vertically.
 12. A furnace as claimed in claim 9 wherein said depression is annular in form and whose collected melt is adapted to electrically connect at least two said long arc transferred columns.
 13. A furnace as claimed in claim 9 wherein said electrical circuit means provides a multi-phase A.C. source and includes electrical connection of an internal electrode of each said torch being utilized with a separate phase of said source and electrical connection of a nozzle portion of each said torch with an adjacent phase of the source via an electrical choke whereby a potential may be established between said internal electrode and said nozzle of each said torch to initiate said pilot arc, and whereby said pilot arc may be transferred to a proximate raw metallic material to establish a direct phase-to-phase potential between at least two torches having main arc plasma columns in electrical contact with the said raw metallic material. 